This invention relates to nuclear reactors and more particularly to control rod guidance apparatus.
In nuclear reactor designs that are well known in the art, the reactor includes a reactor vessel having an inlet and an outlet with a closure head disposed on the top thereof. A core disposed within the reactor vessel consists of an array of fuel assemblies and control assemblies which produce heat in a commonly understood fashion. The fuel assemblies comprise an array of fuel elements which may be cylindrical metal tubes sealed at both ends and containing nuclear fuel while the control assemblies comprise fuel elements and guide thimbles capable of accommodating control rods for controlling the reactivity level of the reactor. The control assemblies are interspersed among the fuel assemblies. The fuel assemblies and control assemblies are supported at their lower end by a lower core plate and at their upper end by an upper core plate while a cylindrical metal member with an open top and bottom, known as a core barrel, surrounds the array of assemblies. The outer surface of the core barrel together with the inner surface of the reactor vessel define an annular passage therebetween. The lower core plate is supported within the reactor core by a support structure attached to the inner wall of the reactor vessel in a manner to support the core within the reactor vessel while the upper core plate is suspended above the core by support columns that extend downward from an upper support plate which is attached to the closure head. The upper support plate and upper core plate define an outlet plenum therebetween. Control rod guide tubes are attached to the upper core plate at locations corresponding to the location of the control assemblies in the core so that control rods may be guided by the guide tubes into the control assemblies.
During reactor operation, a coolant is circulated through the reactor vessel passing in heat transfer relationship with the fuel assemblies and control assemblies thereby removing the heat produced by the assemblies and carrying the heat to electrical generating equipment as is well known in the art. The coolant enters the reactor vessel through the inlet nozzle, flows downwardly through the annulus between the core barrel and the reactor vessel and into the plenum located below the reactor core. The coolant then flows upwardly through the core and in and around the fuel assemblies and control assemblies. The portion of the coolant flowing through the fuel assemblies continues to flow upwardly into the outlet plenum while the portion of the coolant flowing through the control assemblies flows upwardly into the guide tubes and exits the guide tubes radially through side openings in the guide tubes, thus flowing into the outlet plenum. The coolant then exits the reactor vessel through the outlet nozzle.
While the side openings in the guide tubes permit the coolant to exit the guide tubes, the radial flow of coolant produces turbulence that inhibits the movement of the control rods. Since one of the more important purposes of the guide tubes is to provide guidance for the control rods during their travel through the outlet plenum, it is necessary to avoid such turbulent flow that would hinder the operation of the control rods.
In U.S. Pat. No. 3,940,311 to E. Frisch et al, issued Feb. 24, 1976 and assigned to the assignee of the present invention, there is described the use of guide tubes located in the outlet plenum of a nuclear reactor that are used to guide control rods. While the patent to Frisch incorporates the use of side openings in the guide tubes, it does not address the problem of limiting the turbulence in the area near the control rod guidance system.